1. Field of the Invention
This invention relates to a method and apparatus for the multiplexing of plural parallel digital channels having different primary rates into a digital frame having predetermined duration and period, and reciprocally, to a corresponding demultiplexing, method and apparatus.
2. Description of the Prior Art
To multiplex such digital channels in the current state of the art, the latter are multiplexed bit by bit or byte by byte, or more usually word by word of a given number of bits, into a frame having a predetermined high bit rate that is greater than the highest common multiple of the primary bit rates. The rate is constant throughout the frame. The frame is only divided into sectors for frame management purposes, in order to insert management data at regular intervals, notably concerning the quality of transmission of the frame and data concerning the state of at least one digital channel.
Irrespective of the primary rate of a terminal station receiving such a frame produced by a central station servicing plural terminal stations, the terminal station must be matched for transmission and reception to the sole on-line high rate of the frame. The terminal station must comprise a frame demultiplexer in order to reach the hierarchical level of the digital channels and to extract the respective digital channel.
Another solution for distributing digital channels with different rates from a central station is to physically distinguish transmission means intermediary between the stations as a function of the rate of the channels. In this case, a star transmission network with an on-line rate corresponding to the channel rate is associated with each group of terminal stations connected to digital channels having the same rate. When a new terminal station rate becomes available, a new network must be created.